Leaf damages by leaf folders in the first 40 days after sowing that trigger farmers to start their spray routines. Photo credit: KL Heong

In our recent visit to Tan Tru district in Long An province, Vietnam, we conducted a focus group discussion with 50 farmers from a village which had severe hopperburn and virus infestations last season. Farmers described how they started spraying in the early crop stages for leaf folders (sau cuon la) and continued spraying multiple times for planthoppers. Several farmers even sprayed the neighboring habitats thinking that these were planthopper refuges. The most common insecticide used was “Dragon” which contains chlorpyrifos ethyl and cypermethrin.

Fig. 1. Farmers’ insecticide use in Long An province from 1994 to 2008

In 1994 the “no early spray” campaign to reduce farmers’ sprays for leaf folders in the first 40 days after sowing launched in Long An province, reduced insecticide use by 53% (Escalada et al 1999). However none of the farmers we met could recall the campaign or knew that leaf folders in the early crop stages need not be sprayed. This is probably to be expected as the campaign was conducted 14 years ago and over this period, farmers insecticide sprays had remained low for about 8 years and lately has risen substantially (Figure 1). Average yields in the province has remained quite stable. Rice plants have high compensatory abilities and can easily recover from leaf damages at early crop stages. The early season insecticides instead do more harm to the ecosystem services and increase the vulnerability of the crop to invading planthoppers. In this post, we would like to explain the mechanisms involved.

Farmers often respond to visual cues, like leaf folder damages (see picture 1), by spraying insecticides or in some cases follow a prophylactic routine. Common insecticides used include pyrethroids, organophsopates and organochlorines with high toxicity. Such compounds not only kill leaf folders, but predators, parasitoids and aquatic detritivores. In fact the aquatic fauna is most severely affected because all the spray drips are collected in the water. The sprayers farmers commonly use have poor spray nozzles that deliver large spray droplets that will rapidly slip off the leaves into the water. Detritivores are important components of the ecosystem as they provide food for predators (detritivore shunt). When insecticide pressure in IRRI farm was reduced by 95%, species biodiversity of detritivores increased more than 5 folds (Heong et al 2007). These sprays, usually applied in the first 40 days after sowing, also have detrimental effects on predators and parasitoids, thus markedly reducing their biodiversity and abundance and the ecosystem services they provide. The two most important regulatory services affected are pest invasion resistance and pest regulation. Both predator and parasitoid biodiversity doubled in IRRI farm when insecticide pressure was reduced (Heong et al 2007). Besides interfering with the functional biodiversity, these early season sprays reduce the food web structure and disorganize the food chain lengths reducing this from 3, a healthy condition, to 2, an outbreak condition (Heong and Schoenly 1998).

Fig. 3. Ecosystem services are in place at the beginning and remain strong throughout the crop.

Fig. 4. Early sprays then to destroy ecosystem services making the crop vulnerable to invading pests.

Thus rice ecosystems that have received spray disruptions in the early crop stages are placed in a vulnerable condition, lacking abilities to resist invading planthoppers or regulate their population. Should planthoppers invade these crops, there is a high tendency for the planthoppers to develop exponentially leading to an outbreak (illustrated in Figures 3 to 5). Planthopper invasions are random and we often see hopper damages in pockets and not uniformly distributed. In most cases, fields sprayed early may be vulnerable but “escape” hopper damages because hopper invasions may be low of none at all. Spraying for leaf folders in the early crop stages with highly toxic pesticides put the crop at risk to invading hoppers and thus do more harm than good.

Similarly such early season sprays can also cause heavy late season leaf folder attacks. Again leaf folders are also invading species, like planthoppers and will do very well in crops where ecosystem services are compromised (i.e. when food chain length is closer to 2). Farmers who continue to adopt the “no early spray” heuristic will be much better off and there is need to conduct motivational campaigns more frequently to continuously remind farmers about the bad effects of early season sprays.

“Neonicotinoids – from zero to hero in insecticide chemistry” was just reported in 2008 (Jeschke, P. and Nauen, R., 2008). They belong to a new novel class of insecticides that are very potent agonists acting selectively on specific molecular target sites (nicotinic acetylcholine receptors) of insect nervous system; and have made a significant impact on insect pest control. These compounds have low toxicity to mammals, which have a nervous system in which entirely different muscarinic acetylcholine receptors that are unaffected by the neonicotinoids, predominate. Imidacloprid, a potent neonicotinoid, was first commercialized in 1991 and since then has been used extensively especially in the control of insect pests in the agriculture, health and veterinary sectors.

Imidacloprid, like any other insecticide, when used judiciously is beneficial in integrated pest management (IPM) programs. But when used extensively and intensively, it will give rise to many problems such as pest resurgence and resistance, besides eliminating natural enemies and pollinators as well as causing environmental contamination. Here, I trace the development of resistance in the brown planthopper (BPH), Nilaparvata lugens Stål. to imidacloprid.

It was in the late 1990s when the brown planthopper was reported to have developed resistance to orgnanophospate insecticide (Karunaratne, et al., 1999) that rice farmers began to rely more on imidacloprid. In 2006, it was reported that for control of the brown planhopper, imidacloprid has been used for a few years but there was no obvious build-up of resistance in the field population studied; and a field collected strain developed a 250-fold resistance was induced after 37 generations in the laboratory (Liu and Han, 2006). However, over a relatively short period of several years of usage, field resistance had developed against imidacloprid was first observed in Thailand in 2003, and since then it has been found in other Asian countries like China, Japan and Vietnam (Matsumura et al, 2008). Additionally, a test was conducted using two diagnostic doses of imidacloprid on samples collected from China, India, Indonesia, Malaysia, Thailand and Vietnam during 2005 and 2006. It was shown that of 12 samples collected in 2005, only two late season samples collected from India had reduced mortality, while the rest remained susceptible to imidacloprid. But in 2006, all the 13 samples collected had reduced mortality with one of them having a 100-fold resistance compared with the susceptible strain (Gorman et al., 2008).

Toxicity in LD50s of BPH to imidacloprid in IRRI, Philippines, where use is limited and Mekong Delta where use is intensive. Data from KL Heong and Matsumura.

The detoxification of imidacloprid by P450-oxygenases may be an important resistance mechanism in the brown planthopper (Liu et al., 2003); and it is not unexpected. The reason being that insects possess three important classes of enzymes – esterases, oxygenases and S-gluthaione transferases, that specifically cater for the detoxification of a large array of endogenous and exogenous toxic chemicals. It is this process involving esterases that also allows some insect pest species to develop resistance to its own hormone/juvenoids that are applied as insecticides during their control programs. In my opinion, if an insect can develop resistance to its own hormone when applied as a control measure, development of resistance to a particular insecticide/insecticidal toxin is inevitable whenever there is a high selection pressure applied to a natural population of an insect species via intensive and/or prophylactic applications.

As a result of a very high level of resistance developed in field populations of the brown planthopper in China, imidacloprid is likely to be withdrawn for use in field application in 2009. Similar, action will soon follow in other Asian countries when high resistance has been detected. As such, it is unfortunate that imidacloprid, a novel insecticide for the brown planthopper, will soon disappear from the farmers shelves, i.e. “from hero to zero in the control of brown planthopper” in a relatively short span of time.

Policy and institutional decisions should be rational but sometimes they may not be. There is substantial scientific research to show that pests such as the rice planthoppers are induced by insecticides and are symptoms or backlashes of insecticide dependency. These insects are monophagous, highly mobile, have rapid reproductive capacities and well adapted to short term crops. Normally regulated by the diversity of predators and parasitoids that inhibit in and around the rice fields these insects outbreak when the ecosystem’s regulatory services are compromised by pesticides. In the 1970s similar planthopper outbreaks threatened Asian rice production and an Indonesian illustrated the problem using a cartoon (see picture).

A newspaper cartoon illustrating the adaptability of planthoppers.

Huffacker (1971) showed that pesticide addicted agricultural systems generally exhibit symptoms such as pest resurgence, secondary pest outbreaks and insecticide resistance. All these three symptoms now exist in the Mekong Delta. Planthopper outbreaks are caused by either resurgences or secondary developments. Planthoppers in the Mekong are have developed more than 200 fold resistance to some insecticides. Yet in the last 3 years the Ministry of Agriculture and Rural Development (MARD) has released more insecticides than before to rice farmers in response to planthopper outbreaks. For instance in 2006 MARD released an emergency budget of US$ 6.6 million for planthopper management and 55% was used to purchase and distribute insecticides to farmers . Why does science seem to have little impact on policy and institutional decisions?

Herbert Simon (1976) distinguished between substantive and procedural rationality adopted by decision makers. Scientifically produced knowledge contributes to substantive rationality. Such knowledge describes phenomena and explains causal factors. It provides the factual basis for better decisions and satisfies scientific criteria of validity and objectivity. Scientific knowledge is key to deciding on the “best” or the optimal outcome. However most policy and institutional decisions have procedural components and they often displace substantive rationality. In addition policy decisions usually do not focus on optimization or the “best” based on scientific knowledge. Instead they focus on “reasonableness” and what is expected of them in the position they hold.

In February 2009 we conducted a focus group discussion with 15 plant protection practitioners in the Mekong Delta to understand how they make decisions in response of pest outbreaks. It appeared that they rely more on procedural rationality. It is now general knowledge that planthoppers are induced by insecticides and sprays do more harm than good. But the plant protectionists’ main response whenever an outbreak occurs is limited to insecticide distribution as it is expected of them by their bosses as well as by farmers. While the “best” decision based on scientific research is not to apply more sprays as they will worsen the situation, it is not an acceptable decision based on procedural rationality. In fact, a person who will make a decision based on substantive rationality and science will have limited peer support and will even fear losing his or her “chair’ or position. For FGD report, click here.

Plant protection services were established to “protect” crops from pests based on the goals of the 1960s and 1970s when food production was the main objective. Typically plant protection services are structured like fire brigade services, equipped for rapid and mass control. The Tien Giang provincial government for instance would stock enough insecticides to spray 30% of the agricultural production area for emergency mass controls. While there have been numerous papers, discussions, seminars and conferences calling for change in plant protection services to meet the challenges of new pest management environments, these services in Vietnam and indeed in many Asian countries, have remained quite the same. Thus decisions based procedural rationality are limited to mass controls. While such strategies might be useful for some pests, they are ineffective for managing insecticide induced pests, such as the planthoppers, and tend to make the situations worse. Valuable ecosystem services are destroyed rendering rice ecosystems vulnerable to planthopper invasions and with weak abilities to regulate hoppers that succeed in establishing. Read related post here.

Our task is to facilitate bridging the gap between substantive and procedural rationality and incorporating scientific information to improve pest management decisions. The present policy decisions favor insecticide use and cause further deterioration of the ecosystem services. Such decisions are not sustainable and will likely cause environmental pollution and further deteriorations to biodiversity and ecosystem services, like planthopper situations in China. To prevent future planthopper outbreaks, change in policy decision procedures that will use insecticides as a last resort, rather than as first response, will be needed. Tools for facilitating change such as decision support systems and multi agent simulations can be useful, but they will need to incorporate stakeholder participation. Communicative tools such as policy dialogues, consultations (e.g. workshops, informal discussions), extended involvements (e.g. advisory committees, task forces), public information feedback (polls, focus groups, surveys) and joint planning (e.g. partnerships, negotiations) used in non confrontational manner will be expected to play bigger roles.

References cited

Huffaker, C.B. 1971. Biological Control. Plenum Press, NY.

Escalada, M.M. Huan, N.H. and Heong, K.L. 2008. The Brown Planthopper and Virus Problems in Vietnam – A Scoping Study. Proceedings of the Final Consultation Workshop held in Ho Chi Minh City, Vietnam, 8 January 2008. Ministry of Agriculture and Rural Development, Vietnam. CD. Also in http://www.aciar.gov.au/node/8846.